SELECT TOP 1 count(*) as CountSimultaneous
FROM PhoneCalls T1,PhoneCalls T2
WHERE
     T1.StartTime between T2.StartTime and T2.EndTime
GROUP BY
     T1.CallID
ORDER BY CountSimultaneous DESC

查询将为每个呼叫返回同时呼叫的次数.可以从下列命令中选择第一个或SELECT MAX(CountSimultaneous)(作为没有排序的子查询,而不是TOP).